The present invention relates to an oscillator system for producing a jamming signal in a CATV (cable television) system or an intermediate frequency signal in a double superheterodyne receiver.
A system for producing a jamming signal for preventing nonsubscribers from poaching pay television programs is explained as an example.
A subscriber who contracted with a center of a CATV must pay a fee monthly or a fee for every program in order to receive television programs from the CATV. Accordingly, it is necessary to be able to prevent the cheating of pay television programs. There are some scrambling systems where the television signal is processed at the center so that the television signal can not be received through an ordinary television set.
In a jamming method as one of the scrambling systems, an interference wave or noise is inserted in a band of the CATV signal.
FIG. 3 shows a conventional jamming signal producing system. The system is provided in a distributor unit 2b provided out of doors of a home of a subscriber.
A TV signal from a center is applied to a distributor 2a of the system through a transmission line 1 and a directional coupler 2. The TV signal is distributed and applied to individual television sets of subscribers through respective distributor units 2b each having a directional coupler 3 for distributing the TV signal.
The jamming signal producing system has a jamming signal producing circuit comprising a PLL circuit including a voltage controlled first oscillator 6 which produces a first jamming signal. The system further has a second oscillator 7 which comprises, for example, a surface acoustic wave (SAW) resonator to produce a second jamming signal at a fixed frequency of 668 MHz.
The PLL circuit comprises a 1/N divider 14 for dividing the output frequency of the first oscillator 6, a programmable divider 13b, a phase comparator 13c, a low pass filter (LPF) 13d, and the first oscillator 6. The programmable divider 13b is provided to produce a plurality of signals of different frequencies. Each frequency corresponds to a frequency of a channel. The output of the programmable divider 13b is connected to an input of the phase comparator 13c. The phase comparator 13c compares the output of the programmable divider 13b with a reference frequency from a reference frequency generator 13a. The output of the oscillator 6 is connected to a mixer 8 to which the oscillator 7 is connected. The mixer 8 is connected to a combiner 5 through an RF switch 12 and a low pass filter 9. The combiner 5 is fed with the TV signal from the directional coupler 3 through a buffer amplifier 4.
The system has a data signal detector 10 to which the data signal from the directional coupler 3 is fed and a control unit 11 connected to the detector 10. The control unit 11 is operatively connected to the programmable divider 13b and the RF switch 12. In operation, the data signal detector 10 derives a data signal from the TV signal applied from directional coupler 3. When the data signal represents a TV signal of a channel which is not contracted by the subscriber, the control unit 11 applies a control signal to the RF switch 12 to close it. The control unit 11 applies a control signal to the programmable divider 13b so that the programmable divider produces a signal of a frequency corresponding the frequency of the channel. The phase comparator 13c produces an output necessary for producing a control voltage corresponding to the channel to be scrambled. The oscillator 6 accordingly generates a first jamming signal. On the other hand, the second oscillator 7 generates a second local signal of a fixed frequency. The first jamming signal and second local signal are mixed by the mixer 8 to produce a second jamming signal which is fed to the combiner 5.
The resultant second jamming signal is combined with the carrier signal at the combiner 5 so that neither the video signal nor the audio signal of the scrambled channel can be reproduced. When the TV signal is that of a channel which is under contract with the subscriber, the control unit 11 opens the RF switch 12 so that the channel is not scrambled. Furthermore, when a free of charge program is included in a TV signal, the control unit 11 opens the switch 12 only the period of the free program so that nonsubscribers can watch the program.
Experience has shown that the most effective jamming signal combined with a TV signal by the combiner 5 is a signal, the frequency of which is deflected from the picture-carrier frequency of the TV signal by about 250 KHz to the sound-carrier frequency. For instance, in Channel 2 of the United States CATV system through which pay TV programs are transmitted, the frequency of the TV signal is 55.25 MHz. Hence the preferable jamming signal is 55.5 MHz. For example, since the frequency of the signal from the second oscillator 7 is 668 MHz, the desired frequency of the signal from the first oscillator 6 is 723.5 MHz, thereby providing a final jamming signal of 55.5 MHz which is equal to the difference between the signals.
A TV signal of Channel W of the U.S. CATV is 295.25 MHz. In order to produce a desired final jamming signal of 295.5 MHz, the first oscillator 6 is controlled so as to generate a signal of 963.5 MHz (668+295.25+0.25) MHz.
Considering the stability of frequency and variations in characteristics of the oscillator caused by change in temperature and humidity, it is generally thought that the above-mentioned SAW resonator is the best choice as the second oscillator. However, due to the manufacturing variance and mounting conditions at assemblage of the system, the resonant frequency may vary in a range of .+-.150 KHz. Thus the final jamming signal to be combined with the carrier signal is deviated by 100 to 400 KHz from the desired jamming signal. Although the systems which generate a jamming signal having a frequency deflected from the picture-carrier frequency by 250 KHz has very good scrambling effect, if the jamming frequency deviates from the desired frequency, the scrambling effect cannot be obtained.